The present invention relates generally to the production of fuel gas by the gasification of carbonaceous materials such as bituminous, or sub-bituminous coal in down-flow moving-bed gasifiers and employing catalytic material for enhancing the gasification cycle and, more particularly, to such gasification of carbonaceous materials wherein the catalytic material is recycled in the operation and with the recycled catalyst being uniformly distributed over the carbonaceous material.
Moving-bed gasifiers have a proven capability for the efficient conversion of chemical energy contained in carbonaceous materials such as coal, peat, biomass and the like, into product or fuel gases having relatively high BTU values. A highly efficient conversion of the solid carbonaceous material to fuel gas is achieved by employing a countercurrent flow operation in the gasifier whereby particulate carbonaceous material is converted to char and then ash as it flows downwardly in the gasifier countercurrently to an upwardly rising stream of air or air and steam so that the fuel gases are produced by an endothermic reaction with the carbonaceous material and char at relatively high temperatures. Also, this countercurrent flow operation allows some of the sensible heat in the fuel gas to be returned to the reaction zone.
Recent improvements in moving-bed gasifiers for the production of fuel gas from carbonaceous materials utilize a coal pyrolysis cycle in combination with the coal gasification cycle for increasing the coal conversion efficiency while reducing problems associated with the production of tars, coal agglomeration and caking, and the entrainment of fines in the fuel gas. Such an improved moving-bed gasifier is described in U.S. Pat. No. 5,145,490, issued Sep. 8, 1992, which is specifically incorporated herein by reference. In this patented gasifier, a closed end pressure vessel is provided with a discharge for the fuel gas that is in a location removed from the closed end of the pressure vessel so as to significantly reduce the carry over of tars and fines into the fuel gas. This patented gasifier is also provided with a pyrolyzer coaxially disposed within the pressure vessel where the coal is pyrolyzed in the presence of air before being discharged into the high temperature region of the gasifier near the upper end of the pressure vessel. This solid pyrolyzed material that is discharged from the pyrolyzer is in the form of char and descends through a gasification zone surrounding the pyrolyzer and onto a rotating grate. As this char descends through the gasification zone, steam and air are introduced through the base and top of the gasifier to endothermically react with the hot char to provide the gasification reactions for producing the fuel gas. These endothermic gasification reactions are limited to minimum temperatures of about 1700.degree. F. for bituminous coals and about 1500.degree. F. for sub-bituminous coals and thus limit the chemical conversion efficiency that can be achieved.
While not utilized in the gasification reaction described in the aforementioned patent, catalysts such as potassium compounds including potassium carbonate, potassium acid carbonate, and potassium chloride or other compounds such as sodium carbonate, sodium chloride, or calcium oxide have been previously used in gasification cycles to significantly increase the gasification rates even at endothermic gasification reaction temperatures as low as about 1200.degree.-1300.degree. F. Also, at temperatures of about 1400.degree.-1500.degree. F. an exothermic hydrogen/carbon reaction occurs to form methane which releases heat for continuing to drive the endothermic gasification steam/carbon and CO.sub.2 /carbon reactions utilized for producing the fuel gas at temperatures as low as about 1200.degree.-1300.degree. F. However, the problems associated with using catalysts as in previously known gasification systems such as described in the aforementioned patent, is the requirement for providing a uniform distribution of the catalyst across the bed of coal or char particulates in order to ensure the efficient use of the catalyst and, more importantly, the attendant economic problems associated with the loss of catalyst as it is discharged along with the ash from the gasifier. The separation of the catalyst from this ash creates a significant economic burden upon the utilization of catalyst in the gasification cycle due to the difficulties previously encountered during attempts to efficiently separate the catalyst from the ash.